In an effort to shrink the global AIDS epidemic, the world’s largest gay dating app is changing its software this week to urge millions of users to get frequent H.I.V. tests. Grindr, which claims to have 3.3 million daily users from every country in the world, will send men who opt into the service a reminder every three to six months, and simultaneously point them to the nearest testing site. It will also let clinics, gay community centers and other testing sites advertise for free.